An aneurysm is a localized dilation of the wall of a blood vessel, usually caused by atherosclerosis and hypertension, or, less frequently, by trauma, infection, or a congenital weakness in the vessel wall ["Medical, Nursing and Allied Health Dictionary", 4.sup.th Ed., revision editor, K. N. Anderson, Publ., Mosby-Year Book, Inc., St. Louis, Mo., USA]. Currently, the vast majority of the aneurysms are repaired by the conventional open surgical technique, referred to a aneurysmectomy. However, the mortality rate associated with this procedure remains relatively high (2-15% for elective surgery and between 30-50% for emergency surgery following vascular rupture and as high as 71% for patients above 70 years of age [M. E. Clouse, "Device for Performing Endovascular Repair of Aneurysms", U.S. Pat. No. 5,211,658, May 1993]). Furthermore, a major drawback of the procedure is that due to the nature of the surgical operation, it is generally performed in medical facilities with the necessary sophisticated equipment essential for cardiovascular procedures thereby limiting accessibility.
Morbidity and mortality rates following the conventional repair of a non-ruptured abdominal aortic aneurysms (AAA) remain some of the highest in elective surgery. Although endovascular repair has the potential to reduce these figures, its main benefit would be in reducing both the tangible and intangible costs associated with this type of surgery, see Table 1.
TABLE 1 ______________________________________ Conventional Endovascular ______________________________________ Anaesthesia General Local Invasive Local incision Femoral cutdown Transfusion Yes No Procedure Time .gtoreq.6 hrs. 30-60 min Recovery Time .gtoreq.24 hrs. .ltoreq.6 hrs. Hospital Stay 7-10 days .ltoreq.3 days High Risk Candidates No Yes Male Impotency from Many None Operation ______________________________________
Endovascular repair of AAA will reduce the cost of the procedure to the patient, particularly in terms of medical expense, rehabilitation, morbidity and recovery time.
It is widely generally accepted that JC Parodi in 1990 was the first surgeon to successfully clinically demonstrate the concept of endovascular repair of AAA [S. W. Yusuf and B. R. Hopkinson. "Endovascular Repair of Aortic Aneurysm", Leading Articles, British Journal of Surgery, 82, pp. 289-291, 1995]. Using a balloon expandable stent designed by JC Palmaz and a thin-walled tubular Dacron (polyethylene terephthalate) graft, Parodi galvanized the surgical community and venture capital companies, such as EndoVascular Technologies (Menlo Park, Calif.) to strive to develop improved graft designs and deployment mechanisms.
It is widely accepted that the stenting technique pioneered by Parodi is only suitable where a proximal and distal cuff of at least 20 mm is available for secure stent attachment. However, most substantial aneurysms lack such a cuff, particularly in the region of the renal arteries, therefore, Dr. D. Revelas has proposed to mechanically anastomose (to surgically join two ducts, such as blood vessels, to allow flow from one to another) the endoarterially placed graft.
Despite advances in the diagnosis and treatment of patients with AAA, more than ten thousand people die in Britain every year due to sudden rupture of an aortic aneurysm [A. Berger, "Hard Graft Cuts Out Bypass Surgery", New Scientist, Jun. 10, 1995], correspondingly in the USA, the number of deaths is estimated to be approximately fifteen thousand [W. S. Moore, "Endovascular Grafting Technique: A Feasibility Study", Aneurysms, New Findings and Treatments, Chapter 28, Ed., J. S. T. Yao and W. H. Pearce, Publ., Appleton & Lance, Norwalk, Conn., pp. 333-340, 1994]. In addition to improved screening and diagnostic methods for AAA, endovascular repair aims to reduce this number by enabling successful surgical intervention before the onset of the current critical aneurysm size indicated for operation (.+-.5.0 cm).
Basically, work in the field can be divided into two broad categories, namely; the delivery system and the stenting system.
Referring first to the delivery system, the pioneering work by JC Parodi was largely dependent upon conventional stent deployment using an angioplasty balloon. EndoVascular Technologies, the only company currently to hold U.S. Food and Drug Administration (FDA) approval for clinical trials, utilize a balloon deployment system [Lazarus et al., "Endovascular Grafting Apparatus, System and Method and Devices for Use with Them", U.S. Pat. No. 5,275,622, January 1994]. Theoretically, this technique of deployment is highly accurate and reproducible, however, in practice the stent can shift, rotate or separate from the balloon during the procedure. Moreover, the metallic stents can perforate the balloon either during loading, advancement or deployment of the endovascular device resulting in improper deployment. Marin et al. with their U.S. Pat. No. 5,443,477 [Marin et al., "Apparatus and Method for Deployment of Radially Expandable Stents by a Mechanical Linkage", U.S. Pat. No. 5,443,477, August, 1995] proposed an apparatus for intra-luminal delivery and deployment of an expandable prosthesis, using mechanical linkage similar to that proposed by D. Revelas [D. Revelas, "An Endovascular Anastomotic Device", SA Patent Application, August 1994]. However, even with the most modern manufacturing technologies available it is highly unlikely that a working model of the Marin device can be fabricated, although the concept of mechanical deployment is superior to that of using an angioplasty balloon.
In regard to the stenting system, greater attention has been given to the stenting device by inventors than to the delivery systems. Almost every conceivable biomaterial has been proposed as a suitable stent from resorbable polymeric materials to the very latest shape memory allows.
Generally, the stents can be divided into two main groups:
expandable--these stents rely on either mechanical or balloon deployment. Typical examples are the balloon deployed stainless steel/Teflon.RTM. (polytetrafluoroethylene) EVT device [Lazarus et al., "Endovascular Grafting Apparatus, System and Method and Devices for Use with Them", U.S. Pat. No. 5,275,622, January 1994] and the mechanically deployed Fischell stent [R. E. Fischell, "Stent Having a Multiplicity of Closed Circular Structures", European Patent Application EP 0 669 114 A1, August 1995]. (Fischell also acknowledged that in addition to stainless steel, tantalum and titanium, stents fabricated from shape memory alloys could be used). In addition to patents filed for metallic materials, Stack and Klopovic [R. S. Stack and Z. P. Klopovic, "Absorbable Stent", U.S. Pat. No. 5,306,286, April 1994] filed for a device fabricated from a bioresorbable polymer. The device was either balloon or mechanically deployed, once in position tissue ingrowth and encapsulation takes place (as with a Teflon.RTM. or Dacron.RTM. graft), thereafter, the stent is absorbed, reducing the likelihood of embolization. Yet another variation was proposed in 1995 by Dayton [M. P. Dayton, "Minimally Invasive Bioactivated Endoprosthesis for Vessel Repair", U.S. Pat. No. 5,449,382, September 1995] who developed a device fabricated from either a metal or polymer which is then coated with a bioactive substance, such as heparin, hirudin, antithrombogenic or antifibrosis agents, in order to establish a chemical equilibrium with the surrounding tissue. This particular device has a novel locking mechanism to inhibit collapse of the device after deployment. PA1 self-expandable--these devices tend to be mainly variations on the theme of mechanically constricting the stent by twisting and allowing the coiled device to spring open on deployment. Normally, the materials of choice are from the family of stainless steels. Once the correct diameter wire has been decided upon, it is usually woven into a zigzag shape. Typical examples of this type of system were patented by Sung-soon [A. N. Sung-soon, "Intraluminal Stent", European Patent Application EP 0 645 125 A1, March 1995], Fontaine [A. B. Fontaine, "Vascular Stent", U.S. Pat. No. 5,314,472, May 1994] and a prosthesis dependent upon deployment using self-expandable stents was patented by Gunther et al. [R. W. Gunther et al., "An Endovascular Graft Prosthesis and an Implantation Method for Such a Prosthesis", PCT Application WO 95/16406, June 1995].
More recently, alloys which undergo a reversible volume phase transformation in the temperature range -50 to 110.degree. C. commonly termed shape memory alloys (SMA's) have been evaluated as potential stent materials. The Ni--Ti shape memory alloy favored as a stent was developed by the U.S. Naval Ordinance Laboratories and called nitinol. The shape memory effect is one of the most intriguing phenomena in materials science. Even SMA's that have undergone quite severe distortions such as winding into a coil can be easily straightened again by the application of a small amount of heat. As the material is warmed it "remembers" its initial shape and springs back. This effect has not escaped the attention of inventors and once the Ni--Ti alloys were shown to be biocompatible numerous patents were filed utilizing this phenomenon for stent deployment. One of the first patents registered which depended on the shape memory effect for deployment was filed by Clouse [M. E. Clouse, "Method and Device for Performing Endovascular Repair of Aneurysms", U.S. Pat. No. 5,211,658, May 1993] in 1993. Patented variations include woven shape memory alloys [S. S. Lam, "Expandable Stents and Method for Making Same", European Patent Application EP 0 662 307 A1, July 1995; L. Lilip et al., "Self-expandable Stent and Stent-graft and Method of Using Them", PCT Application WO 95/26695, October 1995] (as is customary in this type of patenting these patents are not exclusively related to SMA alloys but they also refer to woven devices fabricated from a broader range of biocompatible materials).
In August 1995, Mori [K. Mori, "Stent for Biliary, Urinary or Vascular System", European Patent Application EP 0 666 065 A1, August 1995] patented the use of a SMA for use in the biliary, urinary or vascular system, what is particulary elegant about this filing is that the Austenite finishing temperature (A.sub.f) ranges from 41 to 43.degree. C. which corresponds approximately with body temperature. Therefore, the stent is capable of assuming a small diameter at temperatures below A.sub.f and an expanded configuration above or equal to A.sub.f.
Recently, Bruin et al. [P. Bruin et al., "Intravascular Polymeric Stent", PCT Application WO 95/26762, October 1995] have proposed using polymeric materials based on either a) amorphous, non-crystallizable polylactic acid networks, b) highly cross-linked polyurethane networks, or c) conversion products of star prepolyesters and di-isocyanate, which exhibit shape memory effects when heated above the glass transition temperature (T.sub.g).
Trout [H. H. Trout, "Aortic Graft, Implantation Device, and Method for Repairing Aortic Aneurysm", U.S. Pat. No. 5,207,695, May 1994], in U.S. Pat. No. 5,207,695, discloses an aortic graft for performing an aortic anastomosis. This is achieved using a series of barbed hooks which pierce the graft and aorta wall in order to stabilize the graft. However, Trout uses a balloon catheter as the deployment mechanism which is different to the mechanical deployment device of the present invention and suffers from all the problems inherent with a balloon catheter.
1. Any balloon device is essentially concentric, particularly if associated with a stent of any known sort. The present invention in a preferred embodiment has individual arms, each of which can take up an independent position (within limits), in an eccentric vessel. No stent is present to limit lateral conformity with an eccentric vessel. The mobility of the graft material determines the conformity of the graft to the vessel wall between the stapling devices.
2. Once stapled to the vessel wall, should the vessel itself continue to dilate (as is often the case with progressive aneurysmal disease), a stapled graft promises better compliance and adherence to the vessel wall with progressive dilatation as compared to a graft held in place by a rigid stent (this theme could be expanded).
3. Stents of any form, either self-expanding or balloon deployable, require a finite length, typically 2 cm, for secure purchase. The stapled anastomosis could conceivably be positioned in as little as 0.5 cm aortic necks with accurate localization.
4. Stapled device does not apply continuous pressure to the inner wall of the host artery, a possible disadvantage of fixed and self-expanding stents which require expansion to at least the stretched diameter of the host artery.
The potential advantages of endovascular repair of AAA has been brought close to every day realization due to advances in catheter delivery systems, fabric and stent technologies, and radiologic imaging [R. M. Green and T. A. M. Chuter, "Experimental Study of Endovascular Grafting for Aortic Aneurysm", Aneurysms, New Findings and Treatments, Chapter 27, Ed., J. S. T. Yao and W. H. Pearce, Publ., Appleton & Lance, Norwalk, Conn., pp. 325-332, 1994]. However, endovascular therapies will require extensive animal testing in order to demonstrate safety, efficacy and effectiveness. The most important criteria for a model to test endovascular graft placement is anatomic correctness. Unfortunately, no such model exists. There are, however, several small and large animal models that have been used to evaluate endovascular grafting.
Mirich et al. [D. Mirich et al., "Percutaneously Placed Endovascular Grafts for Aortic Aneurysms: Feasibility Study", Radiology, Vol. 170, (3PT2), pp. 1033-7, 1988] in 1988 reported on the use of six mongrel dogs with experimentally induced aortic aneurysms. Generally, the results obtained were favorable with only one dog being lost due to occlusion of both renal arteries due to thrombus formation in the vicinity of the incorrectly deployed distal grafts. Chuter [T. A. M. Chuter et al., "Endovascular Repair of Aortic Aneurysms: Straight and Bifurcated Grafts", Dep. Surgery Univ. Rochester, N. Y., U.S.A., 65.sup.th Scientific Sessions of the American Heart Association, New Orleans, 16-19 November, Vol. 86, No. 4, October, 1992] also reported on the successful use of digs to evaluate both straight and bifurcated grafts. Boudghene [F. Boudghene et al., "Endovascular Grafting in Elastase-induced Experimental Aortic Aneurysms In Dogs: Feasibility and Preliminary Results", J. Vasc. Interv. Radiol., 4, (4), pp. 497-504, 1993] and co-workers described successful animal trials with endovascular grafting using elastase induced aortic aneurysms in the beagle dog. Later work conducted on the dog model and reported by Sayers et al. [R. D. Sayer et al., "Endovascular Repair of Abdominal Aortic Aneurysm: Limitations of the Single Proximal Stent Technique", British Journal of Surgery, 81, (8), pp. 1107-10, 1994] highlighted the need for both a proximal and distal stent for anchoring the graft. Using only a single stent resulted in back-bleeding around the distal end of the graft into the aneurysm sac.
As previously stated, the EVT device was the first to receive FDA surgical protocol approval for clinical evaluation. The first implant of the device was performed by W. S. Moore at the UCLA Medical Center on Feb. 10, 1993 [W. S. Moore, "The Role of Endovascular Grafting Technique in the Treatment of Infrarenal Abdominal Aortic Aneurysm", Cardiovascular Surgery, 3, (2), pp. 109-114, 1995]. Out of a screened population of 69 patients with AAA, ten were selected as suitable for endovascular repair. From these, eight were operated on using the EVT device and protocol and two required conversion to conventional aneurysm repair. None of the patients died as a result of the surgery, however, the long-term results for this new intervention are unknown. Moreover, as with all new techniques, randomized, prospective trials comparing standard therapy with endovascular grafting techniques will be needed [M. L. Martin, "Clinical Application of Endovascular Grafts in Aortoliac Occlusive Disease and Vascular Trauma", Cardiovascular Surgery, 3, (2), pp. 115-120, 1995].
Similar results were also obtained with the EVT device in Australia and reported in The Medical Journal of Australia by May et al. [J. May et al., "Endoluminal Repair of Abdominal Aortic Aneurysms", The Medical Journal of Australia, 161 (9), pp. 541-543, 1994]. Out of 12 patients selected as being suitable for endovascular repair with the EVT device, ten were successfully treated by the protocol whilst two cases were abandoned and treated by conventional open repair. After a mean follow-up period of seven months, no deaths were reported. More recent animal trial results using the EVT device were reported by W. J. Quinones-Baldrich et al. [W. J. Quinones-Baldrich et al., "Preliminary Experience with the Endovascular Technologies Bifurcated Endovascular Aortic Prosthesis in a Calf Model", J. Vasc. Surg., 22, (4), pp. 370-379, 1995], the objective of the trial was primarily to evaluate a bifurcated endoluminal prosthesis for transfemoral placement in the aortoiliac position using the calf model as human simulation. Although successful deployment was regularly achieved where a neck of at least 12 mm in length was present, the calf model was found not suitable for chronic graft evaluation due to its sensitivity to spinal chord ischemia.
Successful results were achieved using the EVT device at the Leicester Royal Infirmary, England and reported by Nasim [A. Nasim, et al., "Endovascular Repair of Abdominal Aortic Aneurysms", The Lancet, Vol. 343, pp. 1230-1, May 14.sup.th, 1994]. An 82-year-old woman with a 5 cm symptomless infrarcnal AAA and suitably screened for the procedure was successfully treated and made an uneventful recovery. Yusuf et al. [Yusuf et al., "Transfemoral Endoluminal Repair of Abdominal Aortic Aneurysm with Bifurcated Graft", The Lancet, Vol. 344, pp. 650-1, September 3.sup.rd, 1994] are also developing an endovascular technique for the treatment of AAA at the University of Nottingham. Once again, through careful screening of patients a high success rate was achieved without haemodynamic compromise or major complications. Their research is currently focused on fabricating thinner grafts and deploying stents in necks under 15 mm in length.
It has been estimated that in a quarter of all patients with stents, blood clots accumulate sometimes reblocking the arteries. Johnson & Johnson, who currently manufacture .+-.75% of the 350,000 stents implanted each year, as well as other major companies, are developing coatings for stents to reduce their thrombogenic properties [A. Coghlan, "Heart Attacks Are Given the Slip", New Scientist, Nov. 4.sup.th, 1995]. Generally, stent coatings can be classified as prosthetic (either passive or active) or biological. Typical examples of passive coatings are those which increase the hydrophobicity, such as pyrolytic carbon and urethanes. Active coatings include those which incorporate drugs, such as low-molecular weight heparin, which are slowly released to inhibit thrombus formation. Lastly, biological coatings are represented by seeding of endothelial cells over the stent before deployment [J. C. Palmaz, "Intravascular Stents: Tissue-Stent Interactions and Design Considerations", American Journal of Roentgenology, 160, (3), pp. 613-8, 1993; N. Bonzon et al., "New Bioactivation Mode for Vascular Prostheses made of Dacron.RTM. Polyester", Biomaterials, 16, (10), pp. 747-751, 1995].
As previously highlighted, J. C. Parodi is generally regarded as the pioneer surgeon in the field of endovascular grafting. In his latest journal publication Parodi [J. C. Parodi, "Endovascular Repair of Abdominal Aortic Aneurysms and Other Arterial Lesions", J. Vasc. Surg., 21, pp. 549-557, 1995] reports on the results of the endovascular repair of 50 abdominal aortic aneurysms or iliac aneurysms undertaken between September 1990 and April 1994. In forty of the fifty procedures Parodi reported success, even though some secondary treatment was required in six patients. Of the ten failures, four were attributed to early procedural deaths, one late procedural death, and five leaks.
The present invention provides a method and apparatus for effectively implanting a graft in a vessel of a patient.